Double Joist Okay or Triple Joist for Rooflight

Discussion in 'Builders' Talk' started by mosuhu, Jan 13, 2020.

  1. mosuhu

    mosuhu New Member

    Hi Guys,

    Recently joined the forum, not new to getting my hands dirty though ;)

    So we're in the process of doing up the kitchen, and the extension that we had built some 20-25 years ago has a pitched roof, was a bedroom with standard flat ceiling beneath. We have removed this ceiling, relocated the bedroom and are installing a 1600x900 landscaped format rooflight that weighs approx 140kg.

    The rafters used are 5x2, I will install 75mm celotex insulation between and 50mm celotex to the underside, allowing a 50mm gap between insulation and where the underlay is.

    Anyway, since I have some scope due to the insulation build up I thought I'd use 6x2 C24 doubled up bolted every 400mm to support the rooflight rather than tripling up the 5x2's. Where the 6x2 intersect I will use some 25mm rigid insulation, as the 50mm will not be able to go over this.

    The length of the rafter is just under 3100mm.

    I think the above would be adequte to support the rooflight, but would be interested to hear any thoughts.

    Hope the above makes sense.

    Thanks for reading.

  2. Hans_25

    Hans_25 Screwfix Select

    140kg is two people, so use that as a guide!
  3. mosuhu

    mosuhu New Member

    It's not 2 of me, that's for sure :D

    Any other thoughts from anyone?
  4. RShub1

    RShub1 New Member

    I quickly put some numbers to this and think you'll be fine.
  5. mosuhu

    mosuhu New Member

    That's reassuring to know. Thanks very much RShub1, most appreciated. I can order the timber now :)

    Tbh I have another double up joist question too, but this time for a partition wall that's being built on top. Normally I guess you'd just double up the joist, lay the deck and then build the partition, as noted in the NHBC.

    But I want to build the partition and then lay the deck so it's not under the partition, so if need be can be lifted in future. Can I place 2 timbers spaced apart from each other or is it best to go with triple bolted up?

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